While some outsiders support planned vote to gauge opinion over Maduro's moves, government says poll lacks legitimacy.

Polls have opened in Venezuela in an unofficial referendum meant to gauge public opinion over President Nicolas Maduro's plan to rewrite the constitution.

Sunday's symbolic vote, organised by the opposition, has not been approved by electoral officials.

With authorities refusing to greenlight Sunday's vote and pro-Maduro supporters boycotting it, opposition voters look poised to reject the president's proposal to retool the 1999 constitution by electing a super-body known as the constituent assembly on July 30.

The opposition has told supporters to boycott the July 30 poll, which they see as a power-grab move by Maduro.

The Venezuelan leader insists the constitution needs updating to confront an economic crisis and bring peace after months of protests.

In response to the opposition's vote, the government has called for its own nationwide exercise on Sunday, a rehearsal for the July 30 assembly.

Maduro portrayed Sunday's vote as merely an "internal consultation by the opposition parties" with no electoral legitimacy.

"I call on all Venezuelans to participate peacefully in political events tomorrow, with respect for others' ideas, with no incidents. Peace is what I ask," he said.

But Maduro's constitutional referendum has seen tensions rise in a nation stricken by widespread protests and unrest.

Sunday's opposition vote is being held at 2,000 polling stations across the country and in 80 countries for Venezuelans abroad.

Opposition leader, Henrique Capriles, said: "We're expecting 62 percent turnout on Sunday and we could get 11 million people".

Venezuela's total population is around 30 million.

According to Datanalisis, 70 percent of Venezuelans reject Maduro's idea of a constituent assembly.

"The purpose of the referendum is so that the public says what they actually want," Jose Sanchez, referendum organiser, said. "If they reject Nicolas Maduro's plan, or not."

Foreign observers weigh in

Although the referendum has no legal standing, Latin American presidents from Bolivia, Colombia, Mexico and Costa Rica flew to Venezuela at the opposition's invitation to act as observers of the vote, alongside electoral experts from various countries.

"This fraudulent constitutional assembly will create a majority that will shut congress, throw democracy out the window, wipe out state governors and fire the chief prosecutor," said former Bolivian President Jorge Quiroga to AP news agency on Saturday. "Tomorrow, democracy and freedom are in play."

The United Nations has also weighed in.

"We urge authorities to respect the wishes of those who want to participate in this consultation and to guarantee people's rights to freedom of expression, association and peaceful assembly," Elizabeth Throssell, spokesperson for the UN Human Rights Office of the High Commissioner (OHCHR), said.

On Friday, UN Secretary-General Antonio Guterres called for talks between the opposition and government.

Meanwhile, the head of the Organization of American States, Luis Almagro, called on Venezuelans to take part in Sunday's vote "to prevent the definitive collapse" of the country's institutions.